1. Field of the Invention
The present invention relates to a semiconductor device, more particularly to a multilayer conductive line structure or a multilayer interconnecting structure of a semiconductor device.
2. Description of the Related Art
In production of a semiconductor device, such as a large-scale integrated circuit device (LSI) or very-large-scale integrated circuit device (VLSI), aluminum (Al) and aluminum alloy are widely used as the material for the electrodes and conductive lines. The Al alloy includes aluminum-silicon (Al--Si), aluminumcopper (Al--Cu), and aluminum-silicon-copper (Al--Si--Cu).
Al has advantages such as a lower electric resistance, a good adhesion to silicon dioxide (SiO.sub.2), and a small contact resistance to silicon (Si), but has the disadvantage of easy reactivity on (i.e., easily alloying with) Si and gold (Au). During heat-treatment after the formation of an Al electrode, e.g., an annealing step or a hermetically-sealing step of packaging, Si of the semiconductor substrate dissolves (diffuses) into the Al of the electrode, which causes undesirable effects.
For example, in the case of a bipolar transistor having a shallow emitter, absorption of Si into the Al electrode causes a junction to be broken and generates a short-circuit between the emitter and base. This phenomenon is known as the "spike phenomenon". Furthermore, in the case of a Schottky barrier diode, for example, absorption causes its forward voltage to vary. Accordingly, when many Schottky barrier diodes are formed in a semiconductor device, scattering of their properties is caused.
In order to prevent such problems, it has been proposed to form an electrode coming into contact with a Si substrate surface through a contact window in an insulating film into a triple-layer structure. Such a triple-layer structure electrode comprises a first Al layer, a second refractory metal or its alloy layer, and a third Al layer. The refractory metal is titanium (Ti), tungsten (W), molybdenum (Mo), zirconium (Zr), chromium (Cr), hafnium (Hf), niobium (Nb), vanadium (V), nickel (Ni), platinum (Pt), tantalum (Ta), or palladium (Pd). The refractory metal alloy is, for example, TiW. In this case, the second layer serves as a barrier, since the refractory metal and its alloy do not react to either Al or Si. Therefore, the amount of Al absorbing Si is limited to the first Al layer only. Thus, the amount of dissolution of Si is limited, so as to prevent the above-mentioned undesirable phenomenon.
The adoption of the triple-layer electrode prevents overdiffusion of Si of the substrate into the Al layer, which makes it possible to carry out a heat-treatment of a semiconductor device with the formed triple-layer electrode at a temperature up to 450.degree. C. without deterioration of its electrical properties.
However, for automation of the assembly of semiconductor devices, the devices preferably should be able to resist heating of approximately 500.degree. C. At an elevated temperature, such as 500.degree. C., the TiW barrier layer of the above-mentioned triple-layer electrode loses its barrier property, so that Si of the substrate overdiffuses into the third (upper) Al layer to cause the above-mentioned problems.
Furthermore, in the case of a multilayer interconnecting structure of a semiconductor device comprising, e.g., an Al electrode coming into contact with the Si substrate, an interlaminar insulating layer, and an upper Al conductive line connecting with the Al electrode through a contact window in the insulating layer, when the contact window formed in the interlaminar insulating layer is located over the contact region of the Si substrate, the volume of the Al portion on the contact region is largely increased. Therefore, the amount of dissolution of Si of the substrate into the Al portion is also largely increased, so that a junction formed in the Si substrate is very liable to break. In this case, formation of a conventional barrier layer of, e.g., the above-mentioned TiW between the Al electrode and the Al conductive line is effective for preventing the junction destruction. However, when the semiconductor device with the formed multilayer interconnecting structure is heated at approximately 500.degree. C., again, the barrier property of the barrier (TiW) layer is lost, so that electrical properties of the semiconductor device deteriorate.
Accordingly, a titanium nitride (TiN) layer has been proposed as a barrier layer able to withstand an elevated temperature of approximately 500.degree. C. However, the sputter target used for forming the TiN layer is very brittle and weak and may break during sputtering. Furthermore, the cost of the TiN target is very high. Therefore, this proportion lacks industrial practicality.